Many modern day raster scanning optical image systems utilize a rotating polygon scanner, which by the rotational movement thereof causes the reflected light of an incident, collimated light beam to revolve about an axis near the center of rotation of the rotating polygon. Such scanning systems often suffer from the effects of wobble, introduced into the system by inaccuracies in the bearings utilized to impart motion to the polygon and inaccuracies in the grinding and positioning of the actual facets on the faces of the polygon. The wobble effects cause an uneven raster scan which produces irregular locations of the output scan lines whether utilized in an input or output fashion.
It has been proposed that wobble be cancelled by double reflection from the facets of a rotating polygon, such facets being in parallelism with the axis of rotation. See Mussen et al, U.S. Pat. No. 3,897,132. After a light beam normally incident on a facet is reflected by the facet, the light beam is returned to the same facet, by an arrangement of mirrors, for a second reflection. If there are an even number of mirror reflections between facet reflections, wobble introduced at the first facet reflection is cancelled by the second facet reflection. However, in such a system with polygon facets parallel with the polygon axis of rotation, that is, a zero draft angle polygon, bow effects, defined as a deviation of the scan line from a straight line, are inevitable. If the embodiment of the cited U.S. Patent is analyzed, it can be shown that wobble is well corrected over the scan but bow is excessive, causing a displacement of at least three quarters of an inch at the ends of a nine inch scan line.
U.S. patent application, Ser. No. 532,323, now abandoned filed Sept. 15,1983, in the name of the same inventor and assignee of the present application, discloses a polygon scanning system for correcting wobble by double reflection from the active facet, with the wobble correction being achieved without bow effects. Instead of the impinging light beam being transverse to the axis of rotation of a rotating polygon having facets in parallelism to the axis of rotation of the polygon, there is disclosed a rotating polygon with facets having a predetermined draft angle with the axis of rotation of the polygon, and a light beam impinging on the facets. Once again, by two further reflections from flat, fixed mirrors, the light beam is made to reimpinge upon the same facet; correcting wobble, but now with no effects of bow. Three design variables are used to reduce the effects of bow in the system of the previously filed application; namely, the angle of ray incidence at the first facet reflection, the angle of ray incidence at the second facet reflection, and the draft angle of the polygon.
After the second facet reflection the light beam is passed through conventional f-.theta. scan lens optics having appropriate power in the tangential plane, such that the scanning light beam is linearized and focussed at the surface to be scanned. It would be advantageous to eliminate the f-.theta. scan lens optics, the advantage being not only a reduction in the component count, but also, if the function of the f-.theta. scan lens optics could be achieved by mirrors, the entire optical train would consist only of mirrors and thus have the same focus and scan characteristics at any wavelength. Such an all mirror optical train scanner could operate with white light. A white light raster input scanner (RIS) for scanning documents in color must have an apochromatic lens optical system. An apochromatic lens (corrected for three colors) is expensive, whereas an all mirror RIS would be very inexpensive.